Description :

This research proposal focuses on the role of the Southern Ocean in global change and is submitted by BELCANTO (BELgian research on Carbon uptake in the ANTarctic Ocean) an existing interdisciplinary network of biologists, geochemists, and physical and ecological modelers. The objective is to further develop geochemical proxies and numerical tools for assessing and understanding the present-day functioning of the CO2 biological pump in the iron-limited Southern Ocean and predicting its evolution in response to scenarios of increasing atmospheric CO2. The research methodology will involve and combine collection of historical and new field data, laboratory process-level studies and numerical work in order to improve our understanding of the mechanisms controlling the production of key bloom-forming phytoplankton groups of the Southern Ocean (diatoms and Phaeocystis), their sinking rate and biodegradation when exported in the mesopelagic zone (100-1000 m).

Biological process and proxies investigations will be conducted under laboratory-controlled conditions on cultures of key Antarctic phytoplankton (Fragilaria kerguelensis, Chaetoceros brevis, Phaeocystis colonies and free-living cells) and bacteria grown in sub-nanomolar-iron Southern Ocean waters and at low temperature (1 - 4°C). Biological studies will focus on the light and multiple nutrient (Fe/NO3/NH4/Si) regulation of phytoplankton growth and sinking rate; on the carbon and iron control of bacterial degradation of phytoplankton-derived material including the mineralisation of nitrogen and biogenic silica. Proxies investigations will focus on the use of Ba/Sr, f-ratio and isotopic signatures of C, N, Si for tracing phytoplankton and diatom export production and will involve measurements of Ba/Sr uptake and isotopic fractionation by cultured phytoplankton for different growth and decay conditions. On this basis the relevance of each proxy for tracing export production will be assessed and algorithms will be developed for estimating export production from field measurements.

Fieldwork will concentrate on the collection of new data on surface pCO2 and proxies measurements on suspended matter, collected particles in sediment traps and sediment records. These data will be included in a data base collecting existing data on pCO2, chlorophyll a, diatom/Phaeocystis distributions, primary production and relevant proxies in order to reconstruct distribution fields of atmospheric carbon uptake, bloom formation and export production in the contemporary ocean.

Numerical work will involve the upgrading of the biogeochemical model SWAMCO describing C, N, P, Si and Fe cycling through aggregated biological (diatoms, nano-pico-phytoplankton, heterotrophic nanoflagellates, microzooplancton, bacteria) of the surface layer and the subsequent export production at its lower boundary. Upgrading will include the addition of Phaeocystis colonies as explicit state variable and the implementation of a simple description of the phytoplankton-aggregation process. Parameterisation will be obtained from process-level results. A simplified version will be derived from model analysis for further coupling with a fine grid (30-50km) version of a three-dimensional ice-ocean model. The application domain will cover the region southward of 30°S. Initial and boundary conditions for all state variables and atmospheric CO2 will be derived from existing global atmosphere-ocean models. Atmospheric iron supply will be described as direct dust input or upon snow-ice melting. Model runs will be conducted under present-day climatological forcing.

Validation will be performed by comparison of predictions with in situ observations (sea ice concentration, temperature, salinity), reconstructed fields of surface chlorophyll a and primary production derived from SEAWIFS, diatom/Phaeocystis distributions, and exported production estimated by inverse modelling. Finally climate change prospective scenarios with doubling atmospheric CO2 will be conducted with initial/boundary physical and biogeochemical conditions and atmospheric forcing obtained from OBCM’s runs. Results will be analysed with respect to regional and seasonal changes of CO2 sources and sinks. The resulting changes of sea-ice cover, mixed-layer depth and surface circulation will be described and their causes will be investigated by means of sensitivity experiments. Finally, the impact of the resulting changing physical conditions on the behaviour of the biological pump will be assessed.

Methodology and interaction between partners

1. In-vitro experiments with phytoplankton cultures (ULB-VUB-MRAC) to study the factors controlling
(1) growth, sedimentation and decay of phytoplankton (diatoms and Phaeocystis) under saturating and non saturating conditions of light, iron and Si (ULB) and
(2) new production (VUB) and the expression of proxy signals such as Ba, barite (VUB, MRAC), 15N (VUB), 30Si (MRAC).

2. Composing a data base for carbon fluxes (ULg-VUB-MRAC)
pCO2 (ULg):

(1) Collection of data for under-sampled regions and seasons;
(2) Construction of an autonomous pCO2 analysis system for use on board supply ships;
(3) Elaboration of algorithms for the reconstruction of pCO2 distribution fields based on satellite observation (SEAWIFS) of SST and Chl a.
(4) Deduce atmosphere-ocean CO2 fluxes using ERS wind speed data and pCO2 distribution fields.

Carbon export and mineralization flux (VUB - MRAC): Estimation of the carbon export flux and the POC mineralization profile in the mesopelagic water column (100-1000m) using proxies (new production, f-ratio; 234Th-deficit; 30Si of biogenic silica; 15N of suspended organic matter; Ba-barite). These export fluxes are compared with POC and proxy fluxes in sediment traps, sediments and with model results.

Optimization of the Ba-barite proxy (VUB-MRAC):
(1) The co-variation of mesopelagic Ba and POC export is verified;
(2) Identification of the different factors that control the Ba-signal;
(3) Comparison of export production based on mesopelagic Ba with export production based on POC flux in sediment traps;
(4) Reconstruction of the POC mineralization flux and mineralization profile for the mesopelagic water column;
(5) Optimization of existing algorithms relating the mesopelagic Ba signal to export production and develop new ones.

The validated integrated model is used for the estimation of the CO2 mass balance on global, regional and seasonal scales. Sink and source regions are identified as is the sequestration of organic carbon in the deep sea. The model is used to simulate the situation at atmospheric CO2 redoubling.

Expected results

A better evaluation of the role of the Southern ocean in Global Change. This will be achieved as a result of the development and improvement of complementary proxy approaches contributing to the understanding of the biological carbon pump functioning in an iron-limited Southern Ocean. Simulation of the Southern Ocean’s carbon pump functioning at CO2 redoubling.

Partners

Vrije Universiteit Brussel - ANCH: Has expertise for (i) the determination of new production based on 15N isotope dilution methods and the modelling of nitrogen fluxes related with uptake and release; (ii) the application of proxies of new production and export production (Ba-barite; 234Th-deficit; 15N).

Université Libre de Bruxelles - ESA: Has expertise in the field of phytoplankton processes (growth, sedimentation, grazing, bacterial breakdown) and the development of biogeochemical models.

Musée Royal d’Afrique Centrale - Koninklijk Museum voor Centraal Afrika: Has expertise in the field of trace element and isotope proxy analysis and interpretation.

The Users Committee is composed of scientists active in Southern Ocean research that is complementary to the expertise developed within the present network, and who are participating and/or steering international organisations concerned with Global Change.